The use of Spectroscopic Ellipsometry (SE) to non-invasively characterize properties, (such as thickness, composition, morphology and optical constants), of thin films ex-situ is well known. And, while less common, application to real-time in-situ fabrication monitoring and control is also known, particularly in the semiconductor area. Further, it is known that said techniques are directly applicable to investigating sample systems comprised of multiple thin film layers.
Ellipsometry basically monitors a change in Polarization State of a beam of electromagnetism, which polarization state change occurs as a result of interaction with a sample system. Based upon said change in polarization state, sample system characterizing ellipsometric PSI (ψ) and ellipsometric DELTA (Δ), which are defined by:ρ=rp/rs=Tan(ψ)eiΔwhere rp and rs can be complex Fresnel reflectivities for “p” and “s” polarized components, can be determined. It is noted that Rho (ρ) is a complex number defined as the ratio of the reflectivity of “p-polarized” to reflectivity of the “s-polarized” components of a beam of polarized electromagnetic radiation. In polar form, Tan(ψ) corresponds to the magnitude of the reflectivity ratio and (Δ) corresponds to the phase angle introduced between “p” and “s” polarized components by interaction with the sample system. Further, it should be understood that said “p” component is defined as being in the plane of an incident and reflected beam of electromagnetic radiation, which plane also contains a normal to the surface of the reflective surface of the sample system. And the “s” component is defined as being perpendicular to the direction of the “p” component and also parallel to said reflective surface of the sample system.
It should be appreciated that ellipsometry determines a ratio of “p” and “s” polarization component intensity values rather than an absolute intensity value, and that ellipsometry provides phase shift information, (ie. between said “p” and “s” components), which is not available from electromagnetic beam intensity reflection or transmission data, wherein change in “p” and “s” polarization states are not monitored. It is further to be appreciated that said phase shift information is generally very sensitive to properties, (and changes therein), associated with ultra-thin films.
It should also be appreciated that many types of Ellipsometer systems exist which sequentially comprise a source of electromagnetic radiation, a polarizer means for setting a polarization state, a means for supporting a sample system, an analyzer means for selecting a polarization state, and a detector means for receiving electromagnetic radiation and producing a signal which is proportional to its intensity. Typically at least one element in the ellipsometer system is caused to rotate during data acquisition, and said rotating element can be the polarizer means or analyzer means. A problem in applying rotating polarizer or rotating analyzer ellipsometer systems, however, is that ellipsometric DELTA's of 0.0 or 180 degrees are very difficult to measure therewith without use of means such as the J.A. Woollam Co. Autoretarder, (see U.S. Pat. Nos. 5,757,494 and 5,956,145). In that light it is disclosed that a relevant benefit exists where the polarizer means and analyzer means are both held essentially stationary during data acquisition, and instead an additional element, (ie. a compensator), is present and rotated during said data acquisition. The reason for this is that rotating compensator elllpsometers can measure ellipsometric DELTA's over the entire range of 0.0-360 degrees. In addition, rotating compensator ellipsometer systems can measure ellipsometric PSI's over the entire range of 0.0 to 90 degrees. The J.A. Woollam Co. “M2000” (Reg. Trademark), Rotating Compensator Ellipsometer System is described in U.S. Pat. No. 5,872,630 which is incorporated by reference hereinto.
Continuing, of growing importance is the fabrication and application of multiple High/Low Refractive Index Band-Pass and Band-Reject Filters. For instance, Band-Pass Filters which provide very sharp passband cutoff characteristics, (eg. passbands providing a bandwidth of a nanometer or so with combined high and low side transition to cutoff being less than a nm), are typically comprised of up to a hundred or more layers of alternating quarter-wavelength optical thickness high, and quarter wavelength low optical thickness, refractive index materials, said sequence being beneficially interspersed with half-wavelength thick cavities and/or coupling layers. (Note, “optical thickness” is defined as the index of refraction times the physical thickness). Present manufacturing techniques typically control deposition of the layers of alternating quarter-wavelength high, and quarter wavelength low, optically thick refractive index materials utilizing optical transmission data, wherein a cyclic pattern of Intensity Transmission vs. Layer Number “extrema” turning points are used to determine when to change from depositing low to high, and vice-verse, refractive index material. A problem with this approach is that in some physical thickness and/or wavelength ranges said Intensity Transmission “extrema” turning point data can be relatively insensitive to change in thickness of deposited material, particularly where other than quarter-wavelength optical thickness layers are deposited.
A Search of Patents which apply ellipsometry in the fabrication of multiple layer stack systems, (such as multiple High/Low Refractive Index Layers), has identified very little. The following Patents are disclosed, however, as they mention Ellipsometry in a relevant context:
U.S. Pat. No. 6,051,113 to Moslehi, which discusses apparatus and methodology for multi-target physical vapor deposition of multi-layer materials structures;                U.S. Pat. No. 4,793,908 to Scott et al., which discusses multiple ion source method and apparatus for fabricating multilayer optical films;        U.S. Pat. No. 4,934,788 to Southwell, which discusses deposition of gradient index coatings using coevaporation with rate control; and        U.S. Pat. No. 6,104,530 to Okamura et al., which discusses transparent laminates and optical filters for displays.        U.S. Pat. No. 5,091,320 to Aspnes et al. which discusses ellipsometric control of material growth.        U.S. Pat. No. 4,770,895 to Hartley which discusses ellipsometric control of material growth.Articles which discuss application of ellipsometry to real time applications are:        A paper titled “Extension of Multichannel Spectroscopic Ellipsometry Into The Ultraviolet for Real Time Characterization of the Growth of Wide Bandgap Materials from 1.5 to 6.5 ev”, Zapien et al., Mat. Res. Soc. Symp., Proc. Vol. 569 (1999).        “Instrumental and Computational Advances for Real-time Process Control Using Spectroscopic Ellipsometry”, Pickering et al., Int. Conf. on Metrology and Characterization for VLSI Technology, NIST Gaithersburg, (March 1998).        A Reflectance Anisotropy Spectrometer for Real-time Measurements”, Acher et al., Rev. Sci. Instrum. 63 (11), (November 1992).        
Also disclosed is an Article by the Inventors titled “Optical Metrology Roadmap for the Semiconductor, Optical, and Data Storage Industires II”, Johs et al., SPIE Vol 4449, (2001), which is incorporated hereinto by reference.
Even In view of known prior art, there remains need for improved methodology for monitoring and/or controlling fabrication of multiple layer High/Low Refractive Index stacks such as comprise Band-Pass, Band-Reject and Varied Attenuation vs. Wavelength Filters.